The Electron Gun

Thermionic Effect


The Thermionic Effect is a process of generating a stream of electrons by raising a cathode to a high temperature causing it to emit electrons. In the case of the electron gun the cathode is heated in a vacuum and electrons from the cathode undergo thermionic emission.

Electron gun
The electron gun is the entire combination of all the components in the vacuum tube.

These components are:

Positively charged anode,

Filament or heater,

Negatively charged cathode.

The anode takes form as a positively charged tube through which the electrons will pass through into the region beyond it. The cathode undergoes thermionic heating through a filament of a heater and causes electrons to be emitted. The electrons are focussed into a beam through the anode tube and continue through the otherside as a straight stream.

Kinetic Energy Of Electrons
Relating equations for Electrical Field Strength gives:

Fd=QV (E=V/d and E=F/Q)

This relation shows that the work done (Fd) is equal to the work done per unit of charge (V). So the work done on an object of charge (Q) accelerated through a vacuum (through the electron gun) will be equal to the gain in Kinetic Energy (1/2mv2) on the charge.

So the following relationship can be formed:

QV=1/2mv2 or eV=1/2mv2 with e equal to the charge on an electron.

Equating Velocity
The potential difference (V) is between the anode and the cathode. The mass of the electron is m in the equation.

The velocity of the electron passing through the gun can then be firther measured as:

the square root of 2 electron multiplied by the potential difference, divided by the mass of the electron.

Specific Charge
The Ratio e/m is the specific charge on an electron and is equivalent to 1.76x10(11)CKg(-1). The unit is Coulombs per Kilogram. This value appears when the speed of the electron is small compared to the speed of light.

The electric field in a hollow conductor is zero. This means an electron inside a hollow anode will not be accelerated.